As a detector for GC, detectors of various types have been conventionally in practical use, such as a pulsed discharge detector (PDD), and a low-frequency dielectric barrier discharge ionization detector (BID), in which electric discharge is utilized, besides a flame ionization detector (FID), and a thermal conductivity detector (TCD), and an electron capture detector (ECD).
Among the aforementioned detectors, the FID is generally used in order to detect organic substances. The FID ionizes sample components in sample gas with hydrogen flames and detects a resultant ion current. The FID has the characteristic of a wide dynamic range, but has low sensitivity with respect to incombustible gas and inorganic gas, and therefore compounds to be analyzed are limited because the sample is burned in the hydrogen flame to ionize the sample components.
In the PDD, helium gas or the like is excited by electric discharge through the application of high-voltage pulses to electrodes, thereby plasma is generated, and a sample is ionized by utilizing the emission of light (vacuum ultraviolet light and the like) of the plasma. The PDD has high sensitivity with respect to the incombustible gas and the inorganic gas, and is suitable for detecting almost all compounds with which the gas chromatograph is required to cope. However, the dynamic range of the PDD is smaller than that of the FID. The reason is assumed that, in the PDD, electric discharge is unstable and the state of plasma tends to fluctuate (for example, see Patent Literature 1).
In contrast, in the BID, a space surrounded by a dielectric substance is provided, and an alternating-current low-frequency voltage is applied to a discharge electrode formed on the outside of the dielectric substance, thereby plasma is generated in the space. Thus, in the BID, the electric discharge is stabilized, and the fluctuations in the state of the plasma are suppressed, compared with the PDD. The reason is assumed as follows. The discharge electrode and the plasma are separated by the dielectric barrier (synthetic quartz or the like), which prevents the discharge electrode from being exposed to the plasma, and emission of spattered particles or absorbed gas molecules can be prevented. And the plasma generated by the low-frequency voltage (having frequency of about 5 to 50 kHz, and amplitude of several kilo-volts) has low temperature compared with a case where the plasma is generated by a high-voltage pulse (having frequency of several megahertz, and amplitude of several kilovolts) of the PDD, so that the generation of impurities of gas caused by the heating of the inner-wall material of the detector can be suppressed. As a result, the BID excels in stability, and a favorable signal-to-noise (S/N) ratio can be obtained for a long period of time (for example, see Patent Literatures 1 to 5).